Hydrotalcite as Active and Selective Catalyst for Synthesis of Dehydrozingerone from Vanillin and Acetone: Effect of Catalyst Composition and Calcination Temperature on Activity and Selectivity

Background: Dehydrozingerone is a biosynthetic intermediate and structural analog of curcumin. It is a flavoring ingredient and has antioxidant and antimutagenic properties. Dehydrozingerone can be efficiently synthesized from vanillin and acetone by cross-aldol (or Claisen-Schmidt) condensation using heterogeneous base catalysts. Vanillin can be sourced from the valorization of biomass. Green chemistry aspects such as 100 % atom economy and low E factor are very important in any synthesis which could be achieved by use of heterogeneous catalysis. Methods: Hydrotalcites with different Mg: Al ratio were synthesized and calcined in air at 450 oC and 500 oC to understand the effect of catalyst composition and calcination temperature on activity and selectivity in the synthesis of dehydrozingerone from vanillin and acetone. All catalysts were characterized by different techniques such as XRD, FTIR, ASAP, TPD, TEM and SEM-EDXS. Effects of various process parameters were studied to establish reaction mechanism and kinetics. The kinetic model was validated by experimental data. The product was confirmed by GC-MS and 1H NMR. Results: Among all catalysts, hydrotalcite of Mg: Al of mole ratio 3:1, calcined at 500 oC for 6 h (31CHT500) was found active, selective and reusable. Hydrotalcite calcined at 500 oC gave mixed Mg- Al metal oxides. Dehydrozingerone was efficiently obtained with 100 % selectivity at vanillin conversion of 88 % over 31CHT500 at 130 oC after 4 h. A pseudo first order kinetics was found to fit the data well. Conclusion: Catalyst composition and calcination temperature were affecting the activity of catalysts. Temperature, catalyst loading and vanillin to acetone mole ratio were influencing the reaction rate. Maximum yield of dehydrozingerone was obtained at following reaction conditions: temperature, 130 oC; amount of catalyst, 0.015 g mL −1 ; mole ratio of vanillin: acetone, 1:15; autogenous pressure, 10 atm. All species were weakly adsorbed on catalytic sites. The reaction was pseudo first order with apparent activation energy of 9.4 kcal mol-1. Dehydrozingerone was synthesized from vanillin and acetone by a green process.